Thursday, June 10, 2010

The AND gate performs logical multiplication, commonly known as AND function. The AND gate has two or more inputs and single output. The output of AND gate is HIGH only when all its inputs are HIGH (i.e. even if one input is LOW, Output will be LOW).If X and Y are two inputs, then output F can be represented mathematically as F = X.Y, Here dot (.) denotes the AND operation. Truth table and symbol of the AND gate is shown in the figure above.

Truth Table For AND gate

INPUT

OUTPUT

A

B

A AND B

0

0

0

0

1

0

1

0

0

1

1

1

AND Gate using DiodesWe use the same logic levels, but the diodes are reversed and the resistor is set to pull the output voltage up to a logic 1 state. For this example, +V = +5 volts, although other voltages can just as easily be used. Now, if both inputs are unconnected or if they are both at logic 1, output Z will be at logic 1. If either input is grounded (logic 0), that diode will conduct and will pull the output down to logic 0 as well. Both inputs must be logic 1 in order for the output to be logic 1, so this circuit performs the logical AND function.

AND Gate using Transistors

A simple 2-input logic AND gate can be constructed using transistor switches connected together as shown below with the inputs connected directly to the transistor bases.